Predicting PLL reference spur levels due to leakage current

A simple model can be used to accurately predict the level of reference spurs due to charge pump and/or op-amp leakage current in a phased-locked loop system. Knowing how to predict these levels helps pick loop parameters wisely during the early stages of a PLL system design.Quick review of PLLs

The phase-locked loop (PLL) is a negative feedback system that locks the phase and frequency of a higher frequency device (usually a voltage controlled oscillator (VCO) whose phase and frequency are not very stable over temperature and time to a more stable and lower frequency device (usually a temperature compensated or oven-controlled crystal oscillator, (TCXO or OCXO). As a black box, the PLL can be viewed as a frequency multiplier.

A PLL is employed when there is the need for a high frequency local oscillator (LO) source. Example applications are numerous and include wireless communications, medical devices and instrumentation.Figure 1 shows the building blocks of a PLL system used for generating an LO signal. The PLL integrated circuit (IC) usually contains all clock dividers (R and N), phase/frequency detector (PFD) and the charge pump, represented by the two current sources, ICP_UP and ICP_DN.

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Figure 1. Basic Building Blocks of a PLL

The VCO output is compared to the reference clock (the OCXO output here) after both signals are divided down in frequency by their respective integer dividers (N and R, respectively). The PFD block controls the charge pump to sink or source current pulses at the fPFD rate into the loop filter to adjust the voltage on the tuning port of the VCO (V_Tune) until the outputs of the clock dividers are equal in frequency and are in phase. When these are equal, it is said that the PLL is locked. The LO frequency is related to the reference frequency, fREF, by the following equation:The PLL shown in Figure 1 is called an integer-N PLL because the feedback divider (the N-divider) can only assume integer values. When this divider can assume both integer and non-integer values, the loop is called a fractional-N PLL. The focus here will be only on integer-N PLLs, as different mechanisms are at work in fractional-N PLL.

Phase NoiseThe PLL system of Figure 1 acts as a low-pass filter on the reference clock phase noise and as a high-pass filter on that of the VCO. The low-pass and high-pass filter cutoff frequency is defined by the loop bandwidth (LBW) of the PLL. Ideally, the LO phase noise follows that of the reference clock converted to the LO frequency (that is, multiplied by N/R) up to the LBW and subsequently follows the phase noise of the VCO. The PLL IC’s noise contribution elevates the phase noise in the transition area.

Figure 2 is a phase noise plot generated by PLLWizard, a free PLL design and simulation tool from Linear Technology. The figure shows both the total output phase noise (“Total”), and the individual noises at the output due to the reference (“Ref @ RF”) and the VCO (“VCO @ RF”). The IC’s noise contribution can easily be seen in the highlighted area.

Any unwanted signals on the power supplies shown in Figure 1 (V_OCXO, V_CP and V_VCO) can translate into spurious (spurs) on the LO signal. Careful design of these supplies greatly reduces or even eliminates these spurs. Charge pump related spurs, however, are inevitable. But, they can be reduced with careful PLL system design. These spurs are commonly referred to as reference spurs, though reference here does not mean the reference clock frequency. Rather, it refers to fPFD. An LO signal produced by an integer-N PLL has dual sideband spurs at fPFD and its harmonics.

Figure 3 shows the spectrum of a 2.1 GHz LO signal. fPFD is 1 MHz (N = 2100) and the reference clock is 10 MHz (R = 10). The loop bandwidth is 40 kHz. As a side note, it is worth mentioning that the spurious level achieved in this measurement is world-class due to the high performance of the LTC6945, an ultralow noise and spurious PLL IC from Linear Technology.

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Figure 3. Reference spurs of a 2100 MHz LO Signal with an fPFD of 1 MHz Generated Using the LTC6945 PLL IC from Linear Technology along with the UMX-586-D16-G VCO from RFMD

Causes of reference spurs

In steady-state operation, the PLL is locked, and, theoretically, there is no more need to engage the ICP_UP and ICP_DN current sources of Figure 1 during every PFD cycle. However, doing this would create a “dead zone” in the loop response as there is a significant drop in the small-signal loop gain (practically, an open loop). This dead zone is eliminated by forcing ICP_UP and ICP_DN to produce extremely narrow pulses during every PFD cycle. These are commonly referred to as anti-backlash pulses. This produces energy content on the VCO tune line at fPFD and its harmonics. The negative feedback cannot counteract these pulses since these frequencies are outside the loop bandwidth of a properly designed PLL. The VCO, then, is frequency modulated (FM) by this energy content, and related spurs appear at fPFD and its harmonics, all centered around LO.

Between anti-backlash pulses, the charge pump current sources are off (tri-stated). Inherently, the charge pump has some leakage current when tri-stated. Using an op-amp in an active loop filter (such as in Figure 7 shown in PDF here) introduces yet another leakage current source due to the op-amp’s input bias and offset currents. The aggregate of these unwanted currents, whether sourcing or sinking, causes a drift in the voltage across the loop filter and, consequently, in the tune voltage of the VCO. The negative feedback of the loop will correct for this anomaly by introducing a unipolar current pulse from the charge pump once every PFD cycle so that the average tune line voltage produces the correct frequency out of the VCO. The pulses produce energy at fPFD, which also causes spurs to appear centered around LO and offset by fPFD and its harmonics as previously noted.

In integer-N PLLs, fPFD is often chosen to be relatively small because of the system’s frequency step size requirements. This means that the anti-backlash pulse width, especially with the present high-speed IC technologies, is extremely small compared to the PFD period. As such, a large leakage current causes the total charge pump pulses to be unipolar and tends to be the dominant cause of reference spurs. This phenomenon will be examined in more depth.Reference spurs’ effect on system performance

In a particular communications frequency band there are multiple channels that occupy equal bandwidths. The center-to-center frequency distance between two adjacent channels is equal among all channels and is denoted by channel spacing. Due to several factors, it is common to find large variations in signal strength between any two adjacent channels.

A typical scenario in a multi-channel wireless communications system where a stronger channel exists adjacent to the desired but weaker channel is shown in Figure 4. Only one of the LO reference spurs of concern is shown.

In an integer-N PLL, fPFD is usually chosen to be equal to the channel spacing, which means that the reference spurs are positioned at the channel spacing from the LO. These spurs translate all adjacent and nearby channels to the center of the intermediate frequency (fIF) along with the LO mixing the desired channel to the same frequency. These undesired channels, being uncorrelated to the signal in the desired channel, appear as an elevated noise floor to the desired signal and limit the signal-to-noise ratio.

Relationship between leakage current and reference spur levels

The mathematical prediction of a PLL IC’s phase noise contribution is relatively straightforward and can be accurately determined by calculations. However, the prediction of reference spur levels is traditionally believed to be complex. This section derives a method to accurately predict reference spur levels due to leakage current using simple calculations. Two examples using different loop filters will be examined.

Passive loop filter example

A PLL system with a typical passive loop filter is shown in Figure 5 along with a current source denoted I_Leakage to represent the leakage current of the charge pump. Assuming the PLL is locked, I_Leakage reduces the charge held by CP during the time when the charge pump is off. When the charge pump turns on once every PFD cycle, ICP_UP replenishes the charge lost from CP by applying a short pulse of current. Feedback forces the average voltage seen at V_Tune (V_Tune_Avg) to be constant, maintaining the correct LO frequency. Figure 6 depicts this visually.

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Figure 5. A PLL System with a Passive Loop Filter and I_Leakage Representing the Charge Pump Leakage Current

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Figure 6. CP Discharging through I_Leakage and Charging back through ICP_UP every PFD Cycle.

The derivation of the resultant spurs involves some knowledge of loop stability requirements, the first being LBW restrictions. The LBW of the PLL system is designed to be at least 10 times smaller than fPFD,